a) Field of the Invention
The present invention relates to an improved capping board for use to support hanging legs of anodes and cathodes extending within adjacent electrolytic cells.
b) Brief Description of the Prior Art
In the hydrometallurgical industry, it is of common practice to refine metal by electrolysis in electrolytic cells especially designed for this purpose. The metals to be refined are usually conventional metals like copper, zinc, nickel or cadmium, or precious metals like silver, platinum or gold, and others.
It is also of common practice to use metal plates as anodes or cathodes or both. These metal plates weight several hundred pounds. Usually, the metal to be refined, or the metal used to carry the electric current, is in the form of plates of a given thickness, which are provided at their upper end with two laterally extending projections. Such projections facilitate gripping, handling and hanging of the plates on lateral sidewalls of the cells.
In use, the plates which, as aforesaid, can each weight several hundred pounds, are immersed into the cells in parallel relationship and are used as anodes, cathodes or both, depending on the affinity of the metal being refined.
In order to have the electrodes positioned at the exact place, it is of common practice to place a member called “capping board”, onto the top surface of each lateral sidewall of the cells. These capping boards are used to position the plates with respect to each other. They are also used as electric insulators between adjacent cells and/or each electrodes and/or the ground.
In practice, the capping boards are used not only as supports to position the electrodes, but also as supports to avoid damage to the masonry or concrete forming the lateral side walls of the cells during the insertion and removal of the heaving electrodes.
As examples of such capping boards and the way they can be manufactured, reference can be made to U.S. Pat. No. 4,213,842 issued on Jul. 22, 1980 and Canadian patent No. 1,102,737 issued on Jun. 9, 1981 both in the name of Jean L. DUFRESNE. Reference can also be made to the U.S. Pat. No. 5,645,701 issued on Jul. 8, 1997 and Canadian laid-open patent application No. 2,171,412 filed on Mar. 8, 1996 both in the names of Jean L. DUFRESNE and the present inventor, namely, Robert P. DUFRESNE. Reference can further be made to U.S. patent application Ser. No. 10/725,548 filed on Dec. 3, 2003 and to its Canadian counterpart in the name of the present inventor.
As other examples of such capping boards, reference can also be made to U.S. Pat. No. 3,697,404 issued on Oct. 10, 1972 to Peter M. PAIGE and to U.S. Pat. No. 6,342,136 issued on Jan. 29, 2002 to OUTOKUMPU OY.
As aforesaid, the above mentioned insulating capping boards are used to hold the electrodes at very precise positions. They are also used in combination with electrically conductive contact bars whose purpose is to allow electrical connection between the ends of the anodes and cathodes located in the adjacent cells. Thus, the combined use of capping boards and contact bars have the particularity of allowing insulation and distribution of electric current at the same time.
To achieve proper electrical contact with the contact bar, the plates forming the electrodes are provided with support hanging legs externally projecting on their opposite upper ends. Only one side of the legs of each plate is in contact with a contact bar on one side of the cell where it is located. The other leg of the same plate is held onto the capping board located on the opposite side of the cell in such a way as to be insulated. Thus, the capping board per se plays the role of an insulator and has, for this purpose, to be made of material that is insulating.
So far, it has been of common practice to use contact bars of usually triangular round or rectangular of cross-section or of other cross sectional shape, that extends over the full length of a central path made in the corresponding capping board in order to connect altogether all the anodes of one cell to all the cathodes of the adjacent cell.
The problem with such “lengthy” contact bars is that whenever a short circuit occurs, it “affects” all the electrodes which are connected altogether. Then all the electricity is trying to pass by this said short circuit, which induce very high electric current densities at some specific places and induce very low current density at other places.
These high electric current densities create refined copper of very poor quality, which undesirable. Also, this short circuit creates an increase in temperature.
Such causes the temperature of all the metal plates forming the anodes and cathodes to increase and such an increase may be transmitted to the insulating capping-boards, which may then be subject to deformation. Such deformation is unacceptable since it may generate other short circuits that may propagate from one cell to another cell and which may result in the production of a refined metal with major impurity and defects.